It is often desirable for communication channels and various other signal transmission circuits to have a substantially constant or "flat" amplitude response over a particular passband. In the context of the present invention, the "amplitude response" of a communication channel such as a telephone line refers to the amplitude of the attenuation (or gain) of the channel as a function of frequency. Due to the non-ideal characteristics of the electronic components normally available for the design and implementation of signal transmission circuits, unacceptable variations in the amplitude response may be present over the passband. For example, a telephone line used for voice or data communication can commonly have more than 10 dB of variation over its desired passband of 300-3000 Hz, because of the inherent characteristics of the transmission lines.
The term "amplitude equalization" refers to the technique for producing a nearly flat frequency response over a designated passband. The standard telephone industry approach for performing amplitude equalization on a telephone line has been manual adjustment of a filter network on the channel termination card at the customer's facility. This task must be performed when the line is initially installed, when changes in the line are made, or when complaints about the quality of communications are made. Although this has generally been an acceptable procedure, it requires a skilled craftsperson at the customer's premises to make the adjustments, a time consuming and costly operation. Manual procedures of this type have also introduced the possibility of human error. While automatic amplitude equalization could in principle be performed using computers and complex iterative algorithms, such techniques would not be appropriate for a telephone line communication channel, because the means for performing the equalization must be self-contained on the channel termination circuit card.
The amplitude response of a typical telephone line communication channel, such as the line used to connect an office telephone or data terminal to the central office of a telephone company, is shown in FIG. 3A. There is a wide variation in the specific shape of the response, due to variations in the length of a line, the wire size, the optional use of loading coils, and spur lines. However, in nearly every case of practical importance, the gain of the 300-3000 Hz channel is always highest at the low end and smallest near the high end.
Presently used amplitude equalization networks for telephone line channels are shown in the table set forth in FIG. 1. For each row of the table, the first column shows a network that may be connected in series with the telephone line, the second column shows an equivalent network that may be connected in parallel with the telephone line, and the third column shows the shape of the amplitude response (attenuation or loss vs. frequency) of the networks in columns 1 and 2. These networks are manually installed and adjusted by a craftsperson. Usually two networks are used, one for the low frequency end of the channel, and the other for the high frequency end of the channel. Networks A-D are first order, having only one reactive component, networks E-H have two reactive components and are therefore second order networks. Networks E-H can therefore produce more accurate and sharply defined corrections.
Network B, C, G or H is typically used, along with a resistive load, to increase the loss at frequencies where the amplitude response is too high, i.e., between 300 and 1000 Hz. By installing one of these networks with suitable component values, a loss shape curve can be found that when added to the telephone channel response, results in an approximation of the desired flat amplitude response for frequencies below 1000 Hz. To equalize the upper part of the channel, between approximately 1000-3000 Hz, network F is presently in common use. This network is used as a shunt arm in the equalizer circuit where the signal is fed from a resistive source. Network E will produce similar results when used in the series arm of the circuit. The reactive components of these networks form a resonant circuit tuned to approximately 3000 Hz, where the amount of added loss required is a minimum. The craftsperson, when performing manual equalization, adjusts the value of the resistor. Assuming a constant signal source resistance, the value of the resistance determines the amount of loss variation.